1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to a device for use in irradiating a recording medium having laminated recording layers and servo layers with laser light.
2. Disclosure of Related Art
In recent years, as the capacity of an optical disc has been increasing, the number of recording layers has been increasing. Further, recently, there has been proposed a recording method, wherein laser light is converged on different depth positions in one recording layer. With use of the above method, plural signal layers are formed in one recording layer. This enables to enhance the recording capacity of a recording medium. Examples of such a recording method are a method (micro hologram method), wherein interference of two laser light is used, and a method (two photon absorption method), wherein two photon absorption is used.
In the above recording methods, a servo layer is formed independently of a recording layer to cause laser light to scan along a predetermined trajectory, while converging the laser light at an intended depth position in the recording layer. A guide track is formed on the servo layer and laser light for servo control is converged on the guide track. For instance, recording/reproducing laser light and servo laser light are entered into one objective lens. The objective lens is controlled in such a manner that the focus spot of servo laser light follows a track on the servo layer. With this arrangement, the focus spot of recording/reproducing laser light scans the same trajectory as the track at a predetermined depth position in the recording layer. Adjusting the divergence angle of recording/reproducing laser light at the time of incidence into the objective lens enables to adjust the depth position of the focus spot of recording/reproducing laser light in the recording layer. Thus, the focus spot of recording/reproducing laser light scans the same trajectory as the track at a predetermined depth position.
In the above arrangement, recording/reproducing laser light, and servo laser light have different wavelengths from each other. A servo layer is made of a material having a high reflectance with respect to servo laser light, but having a low reflectance with respect to recording/reproducing laser light. Accordingly, even if recording/reproducing laser light and servo laser light are simultaneously entered into a recording medium, substantially only recording/reproducing laser light is entered into a recording layer.
With the above arrangement, servo laser light reflected on a servo layer, and recording/reproducing laser light reflected on a signal layer are transmitted through one objective lens, and propagate along the same optical path. Accordingly, it is required to provide an arrangement for separating these two laser light, and guiding the separated laser light to corresponding photodetectors.
Further, since plural signal layers are formed in a recording layer in the depth direction of a recording medium, even if recording/reproducing laser light is converged on one (target signal layer) of the signal layers, laser light (stray light) reflected on a signal layer other than the target signal layer may be guided to a photodetector, in addition to the laser light (signal light) reflected on the target signal layer. Thus, in the above arrangement, it is necessary to provide an arrangement for suppressing incidence of stray light into a photodetector for signal light.